Semiconductor packaged devices are continually being designed to be smaller in size. One aspect of manufacturing such devices involves connecting the semiconductor die to the electrically conductive leads to allow the die to communicate with external electrical systems. Typically, electrical wires are connected to these components with a standard stitch bonding (SSB) technique. FIG. 1 illustrates a side plan, diagrammatic view of a semiconductor die 100 that has been connected to a contact lead 102 with a standard stitch bonded wire 104. A capillary 106 is used to bond wire 104 between die 100 and lead 102. Capillary 106 is a hollow tube device through which wire 104 is extruded. Die 100 is shown to be bonded onto a die attach pad 108 with an adhesive material 110. Contact lead 102 is positioned adjacent to die attach pad 108.
FIG. 1 also describes the motion through which a capillary 106 travels in order to bond wire 104 onto die 100 and lead 102. The motion of capillary 106 is described by separating the motion into motion segments A, B, and C. The SSB technique begins with motion segment A where capillary 106 feeds wire 104 out such that an extended length of wire 106 protrudes from capillary 106. Then through various techniques that involve heat and vibration, for example, the extended length of wire 106 is formed into a ball formation. The ball formation, which is at the terminating end of wire 106, is then “ball-bonded” onto a bond pad on die 100. The resulting wire 104 has a squashed ball formation that is attached to die 100, wherein the squashed ball formation is referred to as a ball bond 112.
Then, while feeding wire 104 out of capillary 106, capillary 106 moves through motion segment A by moving upwards from ball-bond 112 and then in a reverse direction relative to lead 102. The motion of capillary 106 in the reverse direction bends wire 104 away from lead 102, which will be the second attachment point of wire 104. Then in motion segment B, capillary 106 again moves in the upward direction and continues to feed wire 104. In motion segment C, capillary 106 moves forward and down until wire 104 is stitch bonded to lead 102. The bend in wire 104 caused by the reverse motion of motion segment A causes wire 104 to have a full arcing shape that arcs upwards between die 100 and lead 102.
The SSB technique used to be a satisfactory technique for bonding wires in semiconductor packaged devices. However, the constraints imposed upon semiconductor device packages today cause SSB to no longer be satisfactory. This is primarily due to the large wire loop height, H1, of wire 104. Height, H1, forces the ultimate thickness of a semiconductor device package to be large since a molding compound is typically required to encapsulate die 100 and the entirety of wire 104.
Attempts to reduce the loop height of a SSB wire involve pulling wire 104 farther downward than normal. In some attempts, wire 104 has been pulled until its loop height, H1, was lower than the heat affected zone (HAZ) of die 100. Unfortunately, the deformed shape of wire 200 caused by the reverse motion of capillary 106 in motion segment A does not hold up well to the stress caused when capillary 106 pulls wire 104 downwards. The reverse motion of motion segment A causes wire 104 to bend towards the right (with respect to FIG. 1), and then the motion of motion segment C causes wire 104 to bend back towards the left. Sometimes bonded wire 104 ends up having the portion of the wire nearest ball bond 112 being deformed and curved to the right and another portion of the wire being deformed and curved back to the left. The resulting wire 104 tends to exhibit necking 114 when force is used to pull wire 104 downwards towards lead 102. Necking 114 occurs in wire 104 near ball-bond 112. Necking 114 refers to fracturing or cracking of wire 104 due to excessive stress imposed upon wire 104. FIG. 2 illustrates a perspective view of two SSB wires 104 that showing signs of necking 114. Necking 114 of wire 104 is exacerbated when wire 104 is pulled further down in attempts to lower loop height, H1.
An alternative technique to lower the loop height of a bonded wire is to reverse the direction of the wire bonding process wherein a wire is first ball bonded onto a contact lead and then stitch bonded onto a semiconductor die. This technique is referred to as reverse stitch bonding (RSB). RSB effectively lowers the loop height because the majority of the wire that forms the loop height rises from the contact lead and along the height of the semiconductor die. Then only a small portion of the loop height actually rises above the die where the wire then continues to extend until it bonds to the die. In other words, a majority of the loop height is hidden within the height of the die.
Although RSB can be effective in reducing the loop height of bonded wires, other problems with RSB reduce the overall desirability of RSB. First of all, long in-bond height consistency is problematic. An in-bond refers to the portion of a wire that extends inwards from the contact lead and over an expanse of a semiconductor die. It is difficult to maintain the desired height consistency of the in-bond portion of bonded wires when using RSB. This is especially difficult when the distance between the first and second bonding points of a wire is large.
A second problem with RSB relates to the formation of a conductive ball on a die bond pad. Such a conductive ball is typically formed on the die bond pad in order to prevent contact and thereby a short circuit between the horizontally oriented segment of the wire that is stitch bonded to the semiconductor die and the die. Contact between the wire and the die is made more possible due to the chances that the size of the conductive ball varies and allows the wires to be positioned closer to the die than is desirable. Contact is also likely because the horizontally oriented region of the wire that is connected to the die may bow downwards onto the die. The conductive ball also represents a manufacturing inefficiency because an extra process step is required to form the ball on the bond pad of the die. Also, an additional stitching motion of the capillary to form the ball is required on top of the motion already required to bond the wire to the lead and the die.
In certain situations, RSB does not provide any advantages. For example, when the top surface of a die and a lead are approximately at the same height or within 3 mils of each other, the RSB does not effectively lower the height of the wire loop. This is because the height of the semiconductor die no longer hides the height of the wire loop. This is especially problematic in Ultra-Thin Packages and Extremely Thin Packages, such as Leadless Leadframe Packages. Also, applications that involve stacked dice tend to involve wires that cross each other. Short circuits are very likely here because the variation of loop heights make it likely that the wires will make contact and short each other out.
In view of the foregoing, there are continuing efforts to provide improved wire bonding techniques to achieve high wire structural integrity and to lower wire loop heights.